This invention relates to image processing techniques and, in particular, a method and system for enhancing an image which is reproduced by a printer as part of a printed sheet.
There are many applications where electronic image reproduction is desirable or even required. In these applications, it is not infrequent that picture quality is such that enhancement of the picture is necessary prior to reproduction. In such a typical application, a 2" by 2" picture forms part of a larger sheet, containing other printed information. It is very important, in reproducing this sheet, to obtain a picture which is clear for purposes of identification. Thus, image enhancing techniques are required to be incorporated as part of the overall system of generating electronic data representing the sheet, in a form suitable for efficient reproduction. However, where large numbers of sheets must be reproduced, computer processing time must be held down. Thus, there is a need for an improved compromise which provides good enhancement at an acceptable cost in terms of computer processing time.
There are a variety of digital processing and reproduction techniques for enhancing images, known in the art. In some systems, data representing respective gray levels is simply shifted, or transformed, in accord with a predetermined formula. The prior art also shows production of a histogram representation of the gray level image data, and various techniques of operating on the histogram data. Generally, the prior art discloses transforming, or mapping pixels at gray levels where most data points are found, into the wider available range of gray levels. Thus, for example, if gray level data is available between 0 and 255, but most of the data is concentrated in a relative peak between 100 and 150, the information between 100 and 150 may be spread out over the entire range of 0 to 255, in accordance with a linear transform or other transformation formula. Reference is made to the following publications which appear in the technical literature, and which disclose various algorithms and techniques for image processing: Digital Image Processing, A systems Approach, William B. Green, Van Nostran Reinhold Company, 1983, pp. 50-60; Handbook of Pattern Recognition and Image Processing, Tzay Y. Young and King-Sun Fu, Academic Press, Inc., 1986; and Algorithms for Graphics and Image Processing, Theo Pavlidis, Computer Science Press, 1982.
While the prior art shows a variety of image processing techniques, the inventors have found a combination of techniques which provides superior enhancement, particularly for reproducing a picture image as part of a larger form, without excessive processing steps. The method of this invention is generally applicable to picture image processing, and can be used in any system involving reproduction of picture images.